Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. HEMTs are preferred in many applications due to their favorable power density, on-state resistance, switching frequency, and efficiency benefits over conventional silicon-based transistors.
HEMTs are typically formed from type III-V semiconductor materials, such as GaN, GaAs, AlGaN, etc. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) forms at the interface between the AlGaN barrier layer and the GaN buffer layer. The 2DEG forms the channel of the device instead of a doped region, which forms the channel in a conventional MOSFET device.
One technique for forming a type III-V semiconductor substrate involves using a silicon wafer as a base substrate. A seed layer is formed on the silicon base substrate, and one or more III-V semiconductor material layers are epitaxially grown on the seed layer. Standard sized silicon wafers that are used in CMOS technology in particular are preferable as base substrates due to their abundance, low cost, and compatibility with standard processing equipment used in silicon fabrication facilities.
One challenge of epitaxially forming III-V semiconductor layers such as GaN on a silicon substrate relates to mechanical stress that arises between the silicon and the III-V semiconductor material. In general, mechanical stress makes the wafers difficult to process, can impact device performance, decrease uniformity of the material properties of the substrate (e.g., doping concentration) and can even lead to complete device failure.
One source of mechanical stress can be attributed to thermal expansion mismatch between the different materials. Epitaxial growth techniques typically involve high temperature cycles. For example, a typical MOCVD (metalorganic chemical vapor deposition) process is performed at temperatures in a range of 900° C.-1200° C. As the substrate cools, the substrate and epitaxial layers contract at different rates due to their different coefficients of thermal expansion.
One way to mitigate mechanical stress attributable to thermal expansion mismatch between the different materials involves incorporating a compensatory stress into the epitaxial layers during epitaxy. However, this produces a curved wafer during epitaxy, which in turn leads to temperature variation in the epitaxial layers across the wafer. This temperature variation impacts various fundamental properties such as alloy composition, doping and material quality. Specially shaped wafer carriers or thicker silicon substrates can mitigate the effect to an extent. However, these techniques are only partially effective, are expensive to implement, and come with other disadvantages.